1. Field of the Invention
The present invention relates to a drying method and a drying apparatus for drying a coating layer, and particularly to a drying method and a drying apparatus for drying a coating layer on a continuous and wide coating surface of a web as a continuously running flexible substrate while at least one liquid composition is applied on the web so as to form the coating layer.
2. Description Related to the Prior Art
E. B. Gutoff & E. D. Cohen teaches in the publication “Coating and Drying Defects” (Wiley-Interscience, John Wiley & sons, Inc) some methods and apparatuses for drying a wet coating layer on a continuous and wide coating surface of a web as a continuously running flexible substrate while at least one liquid composition is applied onto the web so as to form the coating layer. In the method and apparatus for drying, an wind is blown from an air nozzle to dry a wet coating layer on a coating surface of the web while another non-coating surface of the web is supported by rollers, and otherwise an wind is blown from an air nozzle to both coated and non-coating surfaces of the web to float the web in the atmosphere and to simultaneously dry the coating layer on the web in a situation in which the web is not supported by the rollers. The latter one is a non-contact type. In order to efficiently use a space, Japanese Patent No. S48-042903 discloses another non-contact type of a method and an apparatus for drying the wet coating layer, in which a web is coiled on a surface of a cylindrical dryer and an air is blown through holes formed on a wall of the cylindrical dryer. Thus the dry is effectively performed.
Such method and apparatus for drying the wet coating layer with blowing the heated air is called a blow-drying method and a blow-drying apparatus in below. In the blow-drying method, the moisture of the wind is usually adjusted, and thereafter the wind is blown onto the coating surface so as to evaporate the solvent and to dry the coating layer on the coating surface. Although the blow-drying method is excellent in the drying efficiency, the wind is blown to the coating layer directly or through a porous plate, straightening plate, or the like, which causes the bad conditions for the coating layer. Accordingly, the thickness of the coating layer loses uniformity, and unevenness or mura is generated. Further, the convection of the wind makes the evaporation speed of the solvent from the coating layer on coating surface nonuniform. In this case, the coating surface of the web has the orange peel of the coating layer (Ozaki, Yuji “Coating Technology” Asakura-shoten, 1971, Page 293-294). Accordingly, it is hard to form the coating layer having the uniform thickness.
Especially when an organic solvent is contained in a coating solution, the unevenness is generated so much. The reason therefor is as follows: in the primal situation, the organic solvent is contained enough, and when the organic solvent evaporates in this situation, the coating layer has the temperature distribution and a surface tension distribution. In this case, the marangoni convection occurs to generate the unevenness in the coating layer. The generation of the unevenness is a considerable coating defect. Further, when the liquid crystal compounds are contained in the coating layer, it is hard to obtain the predetermined and uniform orientation by blowing the wind to the substrate.
In order to solve these problems, the Japanese Patent Laid-Open Publication No. 2001-170547 (Page 3-5, FIG. 1) teaches a drying method in which a drying apparatus is disposed just after the coating process. In this publication, the drying space in the drying apparatus is separated into plural chambers. In each chamber, a wind is blown in a widthwise from a side to another side of the web while the wind-velocity is controlled. Thus the generation of the unevenness is reduced. The Japanese Patent Laid-Open Publication No. H09-073016 (Page 5, FIG. 5) discloses a drying method in which wire-nettings are provided for the same purpose instead of separating the drying space of the drying apparatus.
Further, there are other drying methods. For example, the concentration of the coating solution becomes higher or the thickener is added to the coating solution to make the viscosity higher, so as to reduce the convection of the coating layer in blowing the wind just after the coating process. Otherwise, when the solvent having high boiling point is used, the leveling effect also reduces the generation of the unevenness when the convection of the coating layer occurs in the wind blown to the coating layer just after the coating process. However, as described in Japanese Patent Laid-Open Publication No. 2001-170547 (Page 3-5, FIG. 1), the above two methods are not adequate for the high speed coating, and increase the drying time, which causes the extremely low productivity.
In order to reduce the nonuniform dry of the coating layer on the coating surface in blowing the wind, Japanese Patent Laid-Open Publication No. 2000-157923 (Page 2-3, FIG. 1) teaches a drying method in which the wind velocity is controlled so as to be lower. Further, Great Britain Patent 1401041, and U.S. Pat. Nos. 5,168,639, 5,694,701 disclose a drying method in which no wind is blown. In the drying method of Great Britain Patent 1401041, the solvent in the coating layer evaporates without wind to perform the drying, and the generated vapor gas is recovered. In this drying method, an entrance and an exit of the substrate is provided in an upper side of a casing of the drying apparatus. In the casing, a non-coating surface of the substrate is heated to promote the evaporation of the solvent from the coating surface for performing the dry. Thus a solvent vapor is generated, and condensed with a condenser which is disposed in a side of the coating surface, so as to dry the coating layer. Further, in the drying method of U.S. Pat. No. 5,168,639, a drum is disposed above the horizontally running substrate so as to recover the solvent vapor. Further, U.S. Pat. No. 5,694,701 proposes a drying method as an improvement of a layout of U.S. Pat. No. 5,168,639.
When a drying apparatus which has a casing surrounding the web is provided just after the coating process of the coating layer, the unevenness is easily generated especially in early dry period while the content of the solvent in the coating layer is large. Therefore it is necessary to make the wind velocity lower in the early dry period as much as possible. Accordingly, it is preferable to prevent the disorder of the wind near to the coating surface, and to keep the concentration of the solvent vapor high in a side of the coating surface of the substrate. Usually, as the solvent vapor of the organic solvent is heavier than the atmosphere, almost of the generated solvent vapor spontaneously diffuses downwards. Accordingly, a method is known, in which an air is forcedly fed such that the solvent vapor of the solvent does not diffuse. Great Britain Patent No. 1401041 teaches that when the coating surface is directed downwards, the downward diffusion of the solvent vapor is promoted, and it is hard to keep the concentration of the solvent vapor high near the coating layer. Therefore the evaporation is not made uniformly. Furthermore, this publication discloses a structure in which the feeding direction is almost the vertical direction. However, the substrate must be inversed for the structure of the disclosed invention, and the zone cannot be made longer.
In the U.S. Pat. Nos. 5,168,639 & 5,694,701, the coating surface of the substrate is directed upward, and the substrate is fed in a almost horizontal direction. In this case, the solvent vapor easily escapes in a widthwise direction of the web, which causes the generation of the unevenness.
Otherwise, in GB Patent No. 1401041, a medium having a high temperature, such as a hot water, is used for heating the non-coating surface of the substrate. Thereby as the medium of the high temperature contacts to or is closed to the substrate, the coating surface of the substrate becomes extremely high, which is preferable in view of promotion of drying. Actually, however, when the temperature of the surfaces becomes too high, the solvent evaporates at a high speed. In this case, the coating layer is often not uniformly dried, or the high temperature decreases the viscosity of the coating layer on the substrate such that the streaming of the solution occurs in the coating layer. Otherwise, when the heating means is not used, the temperature of the coating layer becomes lower by the evaporation of the solvent. Accordingly, the drying speed becomes extremely lower, and plashing occurs in the latter half of the dryer.
The publications of the prior arts disclose the methods of recovering the solvent, but do not teach any concrete methods of reducing the generation of the unevenness just after the drying.